1. Field of the Invention
The embodiments of the present invention relate generally to a coiled tubing injector head utilizing a tension cylinder that automatically adjusts a tension of the chain or conveyor member.
2. Description of the Related Art
In the development and production of an oil or gas well, elongated tubing may be inserted into the well from the surface for such purposes as the injection of certain types of fluids for stimulation of the production, displacing fluids in the well, for performing cleaning operations on the production tubing, as well as various other purposes. A continuous length of tubing is inserted into the well from a large reel at the surface. In the oil and gas industries, this process is known as coiled tubing. An example of an apparatus for inserting and withdrawing coiled tubing into a well can be found in U.S. Pat. No. 5,188,174 to Anderson, Jr. et al., which is hereby incorporated by reference in its entirety.
Coiled tubing units are used for interventions in oil and gas wells, and sometimes the tubing, which comes spooled on a large reel, is used as production tubing in depleted gas wells. The injector head is the heart of a coiled tubing system. Coiled tubing injector heads inject coiled tubing into an oil or gas well to facilitate the servicing of the well.
Coiled tubing injector heads are well known in the art. Coiled tubing injector heads typically have two opposed counter-rotating vertical chains loops with a fixed drive sprocket at the top and a floating sprocket at the bottom. The two opposed counter-rotating chains provide the injector head with the capability to snub coil tubing into a well with pressure. To snub the coiled tubing into a high-pressure well, the injector head must exert a significant amount of compression to overcome the resistance from the wellhead pressure. In other words, the well pressure exerts a force to eject the coil tubing from the well which must be opposed by a force applied to the bottom sprockets to keep the chain loops tight. This is known by a person skilled in the art as chain tension.
The majority of injector head chain and skate bearing failures are caused by applying an improper chain tension when operating the injector head. In the prior art, chain tension is controlled by an operator adjusting hydraulic pressure at a control panel in tension cylinders attached to lower sprocket shafts. The operator must determine the proper chain tension pressure based on the coil tubing's outside diameter and the well pressure (and in horizontal wells, the friction force acting on the coil tubing). These factors can change during the course of a drilling project, requiring the operator to monitor the snub load and chain tension pressure. It is sometimes difficult for an operator to continuously monitor the snub load. As a result, mechanical stops were added at the lower sprocket shafts in the prior art. The mechanical stops prevent the lower sprocket from moving past a predetermined setting, and must be manually adjusted as the chain wears. The chain wear (chain length elongation) is caused by pin-bushing wear (when a chain is operating, the outer surface of the pin and inner surface of the bushing rub against one another, wearing little by little). Another consideration is the position in which the chain and the sprocket engage, which fluctuates, causing the chain to vibrate along with this fluctuation. The vibration occurs because there is a pitch length in the chains, where they can only bend at the pitch point. The height of engagement (the radius from the center of the sprocket) differs when the chain engages in a tangent position and when it engages in a chord. A chain tension pressure that will keep the chains tight against a high snub force results in load spikes from the chain chordal action. At least for the reasons provided above, there is a need for an injector head used in coiled tubing systems that automatically adjusts the tension of the chain.